1. Field of the Invention
The present invention relates to the field of hybrid cords for reinforcement of tires, in particular hybrid cords comprising steel and p-aramid filaments.
2. Description of the Related Art
It is known to make hybrid cords for reinforcing tires comprising steel strands wound with p-aramid strands, both with a steel core and a p-aramid core.
U.S. Pat. No. 5,551,498 describes hybrid cords having a core of two p-aramid strands twisted together and an outer layer or sheath of six steel strands or filaments surrounding the core. Also described are hybrid cords consisting of a core consisting of three steel strands and a layer consisting of four p-aramid strands surrounding the core.
U.S. Pat. No. 4,176,705 describe wire reinforcement cords for tires. The cords consist of a p-aramid core, and steel strands comprised of steel filaments twisted together are disposed about the core.
It is known that steel wires have a lower breaking extension than filaments of aromatic polyamide (aramid). As a consequence, prior art composite reinforcing cords can be exposed only to loads at which the cord's extension does not exceed the breaking extension of the steel wire strands. If this limit load is exceeded, the steel wires rupture and the entire load is taken up by the aramid core which in turn immediately exceeds its own breaking elongation and ruptures as well. In other words, known composite cords break when the extension corresponds to the breaking extension of its steel wire component, in spite of the fact that the latter is distinctly below the breaking elongation of the polyamide core.
Such premature breakage of the steel wires is described in U.S. Pat. No. 4,807,680, which discloses a tire including a hybrid cord consisting of a core of multiple p-aramid filaments surrounded by steel filaments that are wound around the core. The proposed solution is to use almost rectangular cross-section wires, which have a higher elongation at break than corresponding regular round cross-section wires. Hybrid cords using rectangular cross-section wires, are also described in U.S. Pat. No. 4,878,343.
A need remains for hybrid cords which make full use of the p-aramid component.